The present invention relates to a system for detecting a fuel characteristic, such as the volatility of the fuel, for an internal combustion engine.
In one conventional example (as disclosed in Japanese Patent Provision (Unexamined) Publication No. H5-195840), in view of the fuel increase quantity required for cold engine enrichment differing depending on the characteristic of fuel (the volatility of fuel), a control system is arranged to decrease the above-mentioned fuel increase quantity to the maximum degree within a range not causing a surge torque to exceed a permissible limit, and by so doing, to prevent the above-mentioned fuel increase quantity from becoming excessive beyond the requirement of the fuel being used.
This conventional system is, however, arranged to apply the result of modification of the enrichment coefficient dependent on the coolant temperature, to all the cylinders. Therefore, this system can deteriorate the engine driveability significantly by modifying the fuel increase quantity beyond an optimum level (minimum level). To avoid this undesired effect, this system must decrease the enrichment coefficient very gradually, and cannot increase the speed of decrease to reach the minimum correction level, so that a considerable time is required until an appropriate level is reached finally.
Even though the fuel injection quantities are modified equally for all the cylinder, the air/fuel ratios of the individual cylinders are not the same because of differences in the injecting characteristics of the fuel injectors and irregular distribution of the intake air, and there is involved a detection error of an air flow meter for measuring an amount of intake air. Consequently, the coolant temperature enrichment coefficient finally obtained from the modification control is not necessarily appropriate for representing the volatility of the fuel.